(Revised Abstract) DESCRIPTION: Developmental of antiviral agents has proved to be difficult due to the close interactions of viruses with host macromolecular machinery and intracellular replication of viruses. As such, few antiviral agents have made it to the commercial marketplace. In particular, antiviral agents have met with very limited success against viruses that are emerging diseases and/or potential biological weapons. We have taken a rational approach to the design of antiviral agents, and designed thioaptamers for use of antiviral agents. Aptamers are nucleic acid molecules that have been selected from random or high-sequence diversity libraries on their ability to tightly bind to the target (such as a protein) of interest. An iterative process known as in vitro selection is then used to enrich the library for species with high affinity to the target. Using combinatorial selection of phosphorothioate oligonucleotide aptamers, we have selected a dithioated aptamer (XBY-6) that selectively binds transcription factor NF-kB with the aim of using this thioaptamer to modulate the host immune response to infection. In addition, we have found that a second dithioated aptamer XBY-S2, that selectively binds to another transcription factor, Activator Protein 1. Using the NIAID Category B pathogen West Nile virus as a model system for flaviviruses we have found that XBY-6 and XBY-S2 will modulate an otherwise lethal challenge of West Nile virus in the mouse model and protect 50-80% of mice from death. The objective of this cooperative grant application is to elucidate the mechanism(s) of action of XBY-6 and XBY-S2, optimize the antiviral activity of XBY-6 and XBY-S2 in mice, and undertake preclinical testing for safety and efficacy.